1. Field of the Invention
The present invention relates to a differential amplifier circuit and more particularly to a differential amplifier circuit which is able to amplify and output a difference voltage between a pair of input voltages.
2. Description of the Related Art
FIG. 4 shows a well-known conventional differential amplifier circuit. In this circuit, a pair of input terminals IN- and IN+ are connected with two ends of a signal source 10. An output voltage Vin of the signal source 10 is superimposed on an output voltage V1 of a DC voltage power supply 11 which is connected with the input terminal IN+, and is applied across the input terminals IN- and IN+.
The input terminals IN- and IN+, via respective resistances R1, are connected with inverting and non-inverting input terminals of an operational amplifier 12. The inverting input terminal of the operational amplifier 12, via a resistance R2, is connected with an output terminal of the operational amplifier 12 and an output terminal 14. The non-inverting input terminal of the operational amplifier 12 is connected with a DC voltage power supply 13 of a voltage V2 via a resistance R3. In addition, the operational amplifier 12 is supplied with a power source and a ground.
The differential amplifier circuit amplifies R2/R1 times the signal voltage Vin which is superimposed on the DC voltage V1, and thereafter superimposes the amplified voltage on the DC voltage V2 so as to output an amplified-and-superimposed voltage Vout.
In the conventional circuit shown in FIG. 4, the DC voltage V1, being superimposed with the signal voltage Vin, must be small enough to meet a condition "0&lt;VA&lt;Vcc" with respect to the power supply Vcc and ground (0V) of the operational amplifier 12. In the above, "VA" denotes a voltage of the non-inverting input terminal of the operational amplifier 12.
For example, in a case of using the above-mentioned differential amplifier circuit as a current sense amplifier circuit for detecting a current charging or discharging a secondary accumulator, since the secondary accumulator outputs a voltage having a large voltage variation, the power supply Vcc of the differential amplifier circuit supplies a stable voltage by using a regulator to stabilize the output voltage of the secondary accumulator. In general, an output voltage of the regulator is lower than that of the secondary accumulator because of a relationship "VA=R2(V1-V2)/(R1+R2)".
However, in a current sense amplifier in which a differential voltage between two ends of a small resistance inserted between a secondary accumulator and a load is amplified by a differential amplifier circuit, respective voltages applied to the two ends of the small resistance are about as high as an output voltage of the secondary accumulator and higher than an output voltage of the regulator. For this reason, the current sense amplifier does not meet the condition "0&lt;VA&lt;Vcc" of the differential amplifier circuit in FIG. 4 and therefore cannot be applied in the above-mentioned current sense amplifier.
Furthermore, in the circuit of FIG. 4, if V1 is not equal to V2, an electric current I (I=(V1-V2)/(R1+R2)) always flows through the respective resistances R1 of the input terminals IN- and IN+, and, as a result, consumed power will be increased.